1. Field of the Invention
This invention relates generally to the field of computer graphics and, more particularly, to high performance graphics systems.
2. Description of the Related Art
A computer system typically relies upon its graphics system for producing visual output on the computer screen or display device. Early graphics systems were only responsible for taking what the processor produced as output and displaying it on the screen. In essence, they acted as simple translators or interfaces. Modem graphics systems, however, incorporate graphics processors with a great deal of processing power. They now act more like coprocessors rather than simple translators. This change is due to the recent increase in both the complexity and amount of data being sent to the display device. For example, modem computer displays have many more pixels, greater color depth, and are able to display more complex images with higher refresh rates than earlier models. Similarly, the images displayed are now more complex and may involve advanced techniques such as anti-aliasing and texture mapping.
As a result, without considerable processing power in the graphics system, the CPU would spend a great deal of time performing graphics calculations. This could rob the computer system of the processing power needed for performing other tasks associated with program execution and thereby dramatically reduce overall system performance. With a powerful graphics system, however, when the CPU is instructed to draw a box on the screen, the CPU is freed from having to compute the position and color of each pixel. Instead, the CPU may send a request to the video card stating xe2x80x9cdraw a box at these coordinates.xe2x80x9d The graphics system then draws the box, freeing the processor to perform other tasks.
Generally, a graphics system in a computer (also referred to as a graphics system) is a type of video adapter that contains its own processor to boost performance levels. These processors are specialized for computing graphical transformations, so they tend to achieve better results than the general-purpose CPU used by the computer system. In addition, they free up the computer""s CPU to execute other commands while the graphics system is handling graphics computations. The popularity of graphical applications, and especially multimedia applications, has made high performance graphics systems a common feature of computer systems. Most computer manufacturers now bundle a high performance graphics system with their systems.
Since graphics systems typically perform only a limited set of functions, they may be customized and therefore are far more efficient at graphics operations than the computer""s general-purpose central processor. While early graphics systems were limited to performing two-dimensional (2D) graphics, their functionality has increased to support three-dimensional (3D) wire-frame graphics, 3D solids, and now includes support for three-dimensional (3D) graphics with textures and special effects such as advanced shading, fogging, alpha-blending, and specular highlighting.
The processing power of 3D graphics systems has been improving at a breakneck pace. A few years ago, shaded images of simple objects could only be rendered at a few frames per second, while today""s systems support rendering of complex objects at 60 Hz or higher. At this rate of increase, in the not too distant future, graphics systems will literally be able to render more pixels than a single human""s visual system can perceive. While this extra performance may be useable in multiple-viewer environments, it may be wasted in more common primarily single-viewer environments. Thus, a graphics system is desired which is capable of matching the variable nature of the human resolution system (i.e., capable of putting the quality where it is needed or most perceivable).
While the number of pixels is an important factor in determining graphics system performance, another factor of equal import is the quality of the image. For example, an image with a high pixel density may still appear unrealistic if edges within the image are too sharp or jagged (also referred to as xe2x80x9caliasedxe2x80x9d). One well-known technique to overcome these problems is anti-aliasing. Anti-aliasing involves smoothing the edges of objects by shading pixels along the borders of graphical elements. More specifically, anti-aliasing entails removing higher frequency components from an image before they cause disturbing visual artifacts. For example, anti-aliasing may soften or smooth high contrast edges in an image by forcing certain pixels to intermediate values (e.g., around the silhouette of a bright object superimposed against a dark background).
Another visual effect used to increase the realism of computer images is alpha blending. Alpha blending is a technique that controls the transparency of an object, allowing realistic rendering of translucent surfaces such as water or glass. Another effect used to improve realism is fogging. Fogging obscures an object as it moves away from the viewer. Simple fogging is a special case of alpha blending in which the degree of alpha changes with distance so that the object appears to vanish into a haze as the object moves away from the viewer. This simple fogging may also be referred to as xe2x80x9cdepth cueingxe2x80x9d or atmospheric attenuation, i.e., lowering the contrast of an object so that it appears less prominent as it recedes. More complex types of fogging go beyond a simple linear function to provide more complex relationships between the level of translucence and an object""s distance from the viewer. Current state of the art software systems go even further by utilizing atmospheric models to provide low-lying fog with improved realism.
While the techniques listed above may dramatically improve the appearance of computer graphics images, they also have certain limitations. In particular, they may introduce their own aberrations and are typically limited by the density of pixels displayed on the display device.
As a result, a graphics system is desired which is capable of utilizing increased performance levels to increase not only the number of pixels rendered but also the quality of the image rendered. In addition, a graphics system is desired which is capable of utilizing increases in processing power to improve the results of graphics effects such as anti-aliasing.
Prior art graphics systems have generally fallen short of these goals. Prior art graphics systems use a conventional frame buffer for refreshing pixel/video data on the display. The frame buffer stores rows and columns of pixels that exactly correspond to respective row and column locations on the display. Prior art graphics system render 2D and/or 3D images or objects into the frame buffer in pixel form, and then read the pixels from the frame buffer during a screen refresh to refresh the display. Thus, the frame buffer stores the output pixels that are provided to the display. To reduce visual artifacts that may be created by refreshing the screen at the same time the frame buffer is being updated, most graphics systems"" frame buffers are double-buffered.
To obtain more realistic images, some prior art graphics systems have gone further by generating more than one sample per pixel. As used herein, the term xe2x80x9csamplexe2x80x9d refers to calculated color information that indicates one or more of the color, depth (z), transparency, and potentially other information, of a particular point on an object or image. For example a sample may comprise the following component values: a red value, a green value, a blue value, a z value, and an alpha value (e.g., representing the transparency of the sample). A sample may also comprise other information, e.g., a z-depth value, a blur value, an intensity value, brighter-than-bright information, and an indicator that the sample consists partially or completely of control information rather than color information (i.e., xe2x80x9csample control informationxe2x80x9d). By calculating more samples than pixels (i.e., super-sampling), a more detailed image is calculated than can be displayed on the display device. For example, a graphics system may calculate four samples for each pixel to be output to the display device. After the samples are calculated, they are then combined or filtered to form the pixels that are stored in the frame buffer and then conveyed to the display device. Using pixels formed in this manner may create a more realistic final image because overly abrupt changes in the image may be smoothed by the filtering process.
These prior art super-sampling systems typically generate a number of samples that are far greater than the number of pixel locations on the display. These prior art systems typically have rendering processors that calculate the samples and store them into a render buffer. Filtering hardware then reads the samples from the render buffer, filters the samples to create pixels, and then stores the pixels in a traditional frame buffer. The traditional frame buffer is typically double-buffered, with one side being used for refreshing the display device while the other side is updated by the filtering hardware. Once the samples have been filtered, the resulting pixels are stored in a traditional frame buffer that is used to refresh the display device. These systems, however, have generally suffered from limitations imposed by the conventional frame buffer and by the added latency caused by the render buffer and filtering. Therefore, an improved graphics system is desired which includes the benefits of pixel super-sampling while avoiding the drawbacks of the conventional frame buffer.
U.S. patent application Ser. No. 09/251,453 titled xe2x80x9cGraphics System With Programmable Real-Time Sample Filteringxe2x80x9d discloses a computer graphics system that utilizes a super-sampled sample buffer and a sample-to-pixel calculation unit for refreshing the display. The graphics processor generates a plurality of samples and stores them into a sample buffer. The graphics processor preferably generates and stores more than one sample for at least a subset of the pixel locations on the display. Thus, the sample buffer is a super-sampled sample buffer which stores a number of samples that may be far greater than the number of pixel locations on the display. The sample-to-pixel calculation unit is configured to read the samples from the super-sampled sample buffer and filter or convolve the samples into respective output pixels, wherein the output pixels are then provided to refresh the display. The sample-to-pixel calculation unit selects one or more samples and filters them to generate an output pixel. The sample-to-pixel calculation unit may operate to obtain samples and generate pixels which are provided directly to the display with no frame buffer therebetween.
In a system with a super-sampled sample buffer and sample-to-pixel calculation units, the samples are calculated and stored at positions or locations in the sample buffer. Sample position information is stored for each sample which indicates the position or location of each sample in the sample buffer. This sample position information is created when the samples are rendered into the sample buffer, and the sample position information is used by the sample-to-pixel calculation unit in selecting samples for filtering. Due to the large number of samples stored in the sample buffer, improved techniques are desired for more efficiently storing position information of the samples in the sample buffer.
The present invention comprises a computer graphics system that utilizes a super-sampled sample buffer and a programmable sample-to-pixel calculation unit for refreshing the display, wherein the graphics system has improved storage of position information of the samples in the sample buffer. In one embodiment, the graphics system may have a graphics processor, a super-sampled sample buffer, and a sample-to-pixel calculation unit.
The graphics processor generates a plurality of samples and stores them into a sample buffer. The graphics processor preferably generates and stores more than one sample for at least a subset of the pixel locations on the display. Thus, the sample buffer is a super-sampled sample buffer which stores a number of samples that, in some embodiments, may be far greater than the number of pixel locations on the display. In other embodiments, the total number of samples may be closer to, equal to, or even less than the total number of pixel locations on the display device, but the samples may be more densely positioned in certain areas and less densely positioned in other areas.
The sample-to-pixel calculation unit is configured to read the samples from the super-sampled sample buffer and filter or convolve the samples into respective output pixels, wherein the output pixels are then provided to refresh the display. The sample-to-pixel calculation unit selects one or more samples and filters them to generate an output pixel. Note the number of samples selected and/or filtered by the sample-to-pixel calculation unit is typically greater than one.
The sample-to-pixel calculation unit may access the samples from the super-sampled sample buffer, perform a filtering operation, and then provide the resulting output pixels directly to the display, preferably in real-time. The graphics system may operate without a conventional frame buffer, i.e., the graphics system may not utilize a conventional frame buffer which stores the actual pixel values that are being refreshed on the display. Thus, the sample-to-pixel calculation units may calculate each pixel for each screen refresh on a real time basis or on an on-the-fly basis.
The graphics processor renders samples into the sample buffer at computed positions or locations in the sample buffer. The sample positions may be computed using various sample positioning schemes, such as a grid-based or stochastic position generation. The graphics processor may operate to calculate positions during rendering of samples into the sample buffer and utilize these calculated positions during rendering, and the graphics processor may also store the calculated position information for each of the samples. Alternatively, the graphics processor may use pre-computed position information. The position information indicates the position or location of the respective samples in the sample buffer. The position information may be programmable, such as on a per frame or per bin basis. The sample-to-pixel calculation unit uses the position information to select the samples for filtering during generation of output pixels.
In one embodiment, for each sample, the position information comprises one or more offset values, such as an x-offset and a y-offset, wherein the offset values are relative to pre-defined locations in the sample buffer. The one or more offset values may be offsets relative to a pre-defined grid in the sample buffer, such as pre-determined pixel center coordinates or pre-determined bin coordinates. For example, the samples may be stored in the sample buffer within bins, wherein each respective bin defines a region in the sample buffer in which samples in the respective bin are located, and the one or more offset values comprise offset values relative to a bin. Thus, the sample-to-pixel calculation unit may determine a position of each sample within a respective bin by using the one or more offset values associated with the sample and the sample""s bin position.
The position information may be stored in the sample buffer with the samples, or may be stored in a separate sample position memory coupled to the graphics processor. In one embodiment, where the samples are stored in the sample buffer within bins, the samples are stored in the sample buffer according to a bin ordering, wherein the bin ordering indicates a position of the samples in the respective bin. In other words, the samples are stored in the sample buffer memory in an order corresponding to their respective relative positions in the bins. Thus, separate storage of sample position information may not be necessary, as the bin ordering of a sample in the sample buffer memory implies a position or offset of the sample. A look-up table memory may be included which stores position information, such as offsets, for each of the samples according to the bin ordering. The sample-to-pixel calculation unit may use the bin ordering of the samples in the bins to index into the look-up table memory to determine the position information of the samples.
In one embodiment, the look-up table memory stores a number of offset values which is less than the number of samples stored in the sample buffer. For example, the look-up table memory may store a number of offset values corresponding to only one bin of the sample buffer, wherein these offset values are reused for each bin of the sample buffer. The sample-to-pixel calculation unit may also manipulate bits in addresses to obtain different offset values for samples in respective bins.
The graphics system may include double buffered sample position memories, e.g., a first memory and a second memory which each are used to store position information for each of the samples. The graphics processor uses the first memory to access and/or store position information during rendering of samples into the sample buffer, while the sample-to-pixel calculation unit may contemporaneously use the second memory during generation of output pixels. Thus, the first memory may transfer current position information for a current frame to the second memory. The sample-to-pixel calculation unit may then use the second memory to determine the current position information of the samples, while the graphics processor contemporaneously stores position information for a subsequent frame into the first memory. Thus, the sample position memory may essentially be double buffered in this manner.